Offshore drilling platforms are typically supported by jackets having a number of legs which are anchored to the sea bed by piles. The jackets are necessarily quite massive, in order to support the platform and to resist the loads caused by waves, wind, currents and tides and by the impact of ships. The great weight of the jackets makes it more difficult, however, to transfer the jacket structure from the point of fabrication to the desired location in the water. As a practical matter some buoyancy of the jacket is required in order to more readily and accurately set the jacket in place on the sea bed.
As a result of the opposing requirements of great mass and buoyancy a large number of various jacket designs have been proposed. Some pertain to the type of jacket which an be fabricated in sections, with the sections being transported to the installation site and connected together there. These structures are often provided with various types of hollow support members which are flooded to provide the necessary ballast to lower the structure into place. Such structures, which can be quite complicated in esign, are generally limited in use to relatively calm waters, such as those encountered in the Gulf of Mexico.
Jackets utilized in rougher waters, such as those encountered in the North Sea, are required to be more massive due to the more severe forces to which the jackets may be subjected during use. Such jackets are more commonly fabricated in their entirety, then either lowered into place by cranes or launched from a transportation barge. Thus the problem caused by the great weight of the jacket becomes more acute, since the lift weight or launch weight is greater and the maneuverability of the jacket while placing it is reduced. For this reason, ways of reducing the weight and increasing the buoyancy of such jackets are constantly being sought.
In one form of jacket construction which makes use of relatively short pile sleeves located at the base of the legs for anchoring the jacket to the sea bed, it has been suggested to reduce the weight by shortening the length of the sleeves even more, theorizing that the lost strength resulting from shorter pile sleeves can be compensated by optimizing the grouted connection bstrength between the pile sleeve and the pile. It was found, however, that such an arrangement requires increased stress resistance in the connection between the jacket support leg and the pile sleeve. Thus both the pile sleeve and the usual shear plate which connects the support leg and the pile sleeve would have to be made thicker to provide greater stress resistance. Since the increased weight of the thicker plate metal would tend to offset the weight reduction achieved by shortening the pile sleeves it was not practical to so shorten the sleeves.
It has also been suggested to provide extra buoyancy to the jacket by constructing buoyancy chambers at the base of the support leg, such as disclosed in U.S. Pat. No. 4,014,176, wherein either foam or an airtight compartment is provided. That structure, however, requires the support leg to be entirely surrounded by the outer wall of the chamber, resulting in the introduction of significant extra weight which tends to offset the benefits of the added buoyancy. This design also requires more construction time and is more expensive to construct.
A simpler way of providing more buoyancy to a jacket without a complicated expensive design would be highly desirable, particularly if it also enables further savings in weight, such as, for exmaple, by permitting the shortening of the pile sleeves as discussed above. It is therefore a main object of the invention to provide additional buoyancy within the parameters mentioned.